1. Field of the Invention
The present invention relates to a receiver for rejecting an image signal, and in particular to a receiver for rejecting an image signal which is capable of removing imbalance generated by gain and phase errors.
2. Background of the Related Art
In a mobile communication system, because a signal frequency carrying information, such as audio, data and picture information, is a very low base band frequency, it has to be modulated into a high frequency signal for transmission and, when the signal reaches a receiver, it has to be demodulated into an initial frequency band. Receivers are largely divided into a Heterodyne type and a Homodyne type, according to the type of RF (radio frequency) signal conversion method used.
In a Heterodyne method, a RF signal inputted through a local oscillator and an intermediate frequency oscillator is converted into a base band signal, and the RF signal undergoes two down conversion processes. In the Heterodyne method, because an image signal is generated, an image filter for rejecting the image signal is required, and thus it is difficult to minimize the size of the receiver.
In a Homodyne method for solving the problem of the Heterodyne method, a received RF signal is directly down-converted into a base band signal. The frequency in which the local oscillator is operated is the same as that of the RF signal, and thus a desired signal can be promptly detected without undergoing a process for converting the RF signal into an intermediate frequency (IF) signal. Because the Homodyne-type receiver does not require components for processing an intermediate frequency, it can be made smaller and simpler and can thus used for a mobile communication terminal.
FIG. 1 is a block diagram illustrating a structure of a general super Heterodyne receiver, which is used in most communication systems.
As depicted in FIG. 1, a RF signal from an antenna is inputted to a LNA (low noise amplifier) 2 through a duplexer 1, where it is amplified and then sent to an image rejection filter 3. The image rejection filter 3 removes an image frequency from the amplified RF signal and sends the RF signal to a RF down-mixer 4. The RF down-mixer 4 down-converts a frequency of the RF signal into an IF band by mixing the RF signal with a low oscillator (LO) frequency supplied from a RF local oscillator 5. An intermediate frequency passes a channel select filter 6 and is amplified in an If, amplifier 7. A low noise-amplified IF signal is mixed with a LO frequency supplied from an IF local oscillator 9 in an IF down-mixer 8 and is down-converted into a base band. The base band signal passes a LPF (low pass filter) 10 (and is extracted as a desired signal.
The image rejection filter 3 is a band pass filter used at an input side of a mixer in order to prevent a performance lowering phenomenon at an output side of the mixer, by removing an image frequency element symmetric with respect to a reference frequency in a frequency mixing process.
A receiver having a Hartley structure and a receiver having a Weaver structure can reject an image signal without using an image rejection filter by applying a SSB (single side band) modulation technique to the Heterodyne method.
A receiver having a Hartley structure rejects an image signal by dividing an input signal into an I-signal and a Q signal, and combining the I-signal with the Q-signal after the Q-signal passes through a phase-shifter. A receiver having a Weaver structure rejects an image signal by using two oscillators, each having a frequency as ω1>>ω2, and a low-pass filter or a band pass filter. Specifically, a receiver having a Weaver structure uses an oscillator having a frequency as ω2 instead of a phase-shifter.
A receiver having a Hartley structure and a receiver having a Weaver structure will now be described in detail with reference to the accompanying drawings
FIG. 2 illustrates a receiver having a Hartley structure, in accordance with the related art. It includes a first mixer 12a for mixing a RF input signal with an output signal from a local oscillator 11; a second mixer 12b for receiving a signal from the local oscillator 11 that has been phase shifted by π/2 by a first phase shifter 13, and mixing it with the RF input signal; first and second low-pass filters 14a and 14b for passing a low band of the signal from the first mixer 12a and the signal from second mixer 12b, respectively; a second phase shifter 15 for phase-shifting an output signal from the second low-pass filter 14b by π/2 and outputting it; an adder 16 for adding the output signal of the second phase-shifter 15 to the output signal of the first low-pass filter 14a, and for rejecting an image signal; a third mixer 18 for mixing an output signal of the adder 16 with an output signal of an intermediate frequency oscillator 17, and down-converting a frequency of the mixed-signal; and a third low-pass filter 19 for outputting a desired signal by passing a low band of an output signal from the third mixer 18.
The operation of a receiver having a Hartley structure will now be described.
A RF (radio frequency) signal inputted to the receiver is divided into an I (cosine)-signal and a Q (sine)-signal by using the first phase shifter 13 and the local oscillator 11. The signals are respectively down-converted into an intermediate frequency in the first mixer 12a and the second mixer 12b. Because a high frequency element of the I signal (A) and the Q-signal (B) respectively outputted from the first and second mixers 12a, 12b is rejected in the first and second low-pass filters 14a, 14b, the down-converted signal as the intermediate frequency and an image signal (C and D) are respectively outputted from the first and second low-pass filters 14a, 14b. 
The Q-signal (D) outputted from the second low pass filter 14b is phase-shifted by π/2 by the second phase-shifter 15, and the phase shifted Q-signal (E) is added to the I-signal (C) outputted from the first low-pass filter 14a, thereby rejecting the image signal. An output signal (F) of the adder 16 is mixed with an output signal of the intermediate frequency oscillator 17 in the third mixer 18, and a frequency of the signal is down-converted. The output signal of the third mixer 18 is filtered through the third low-pass filter 19. The third low-pass filter 19 outputs a base band signal.
When a RF input signal inputted to the receiver is a single tone of ARF cos ωRFt+AIM cos ωIMt, an output of the local oscillator 11 is ALO(cos ωLOt), and output of the intermediate frequency oscillator 11 is AIF(cos ωIFt). The equations below represent the signals at points A–F in FIG. 2:
            A      ⁢              :              ⁢                          ⁢                                    A            RF                    ⁢                      A            LO                          2            ⁢              {                                            cos              ⁡                              (                                                      2                    ⁢                                                                                  ⁢                                          ω                      LO                                                        +                                      ω                    IF                                                  )                                      ⁢            t                    +                      cos            ⁢                                                  ⁢                          ω              RF                        ⁢            t                          }              +                                        A            IM                    ⁢                      A            LO                          2            ⁢              {                                            cos              ⁢                              (                                                      2                    ⁢                                                                                  ⁢                                          ω                      LO                                                        -                                      ω                    IF                                                  )                                      ⁢            t                    +                      cos            ⁢                                                  ⁢                          ω              IF                        ⁢            t                          }                        B      ⁢              :              ⁢                                    A            RF                    ⁢                      A            LO                          2            ⁢              {                                            sin              ⁡                              (                                                      2                    ⁢                                                                                  ⁢                                          ω                      LO                                                        +                                      ω                    IF                                                  )                                      ⁢            t                    -                      sin            ⁢                                                  ⁢                          ω              RF                        ⁢            t                          }              +                                        A            IM                    ⁢                      A            LO                          2            ⁢              {                                            sin              ⁢                              (                                                      2                    ⁢                                                                                  ⁢                                          ω                      LO                                                        -                                      ω                    IF                                                  )                                      ⁢            t                    +                      sin            ⁢                                                  ⁢                          ω              IF                        ⁢            t                          }                        C      ⁢              :              ⁢                                    A            RF                    ⁢                      A            LO                          2            ⁢              (                  cos          ⁢                                          ⁢                      ω            IF                    ⁢          t                )              +                                        A            IM                    ⁢                      A            LO                          2            ⁢              (                  cos          ⁢                                          ⁢                      ω            IF                    ⁢          t                )                        D      ⁢              :                -                                        A            RF                    ⁢                      A            LO                          2            ⁢              (                  sin          ⁢                                          ⁢                      ω            IF                    ⁢          t                )              +                                        A            IM                    ⁢                      A            LO                          2            ⁢              (                  sin          ⁢                                          ⁢                      ω            IF                    ⁢          t                )                        E      ⁢              :              ⁢                                    A            RF                    ⁢                      A            LO                          2            ⁢              (                  cos          ⁢                                          ⁢                      ω            IF                    ⁢          t                )              -                                        A            IM                    ⁢                      A            LO                          2            ⁢              (                  cos          ⁢                                          ⁢                      ω            IF                    ⁢          t                )                  F    ⁢          :          ⁢                  A        RF            ⁡              (                  cos          ⁢                                          ⁢                      ω            IF                    ⁢          t                )            
Herein, ARF is a size of the signal that a user wants to detect and AIM is the size of an image signal.
FIG. 3 illustrates a receiver having a Weaver structure, in accordance with the related art. It includes a first mixer 22a for mixing a RF input signal with an output signal of a first frequency oscillator 21a; a second mixer 22b for receiving a signal from the first frequency oscillator 21a that has been phase shifted by π/2 by a first phase shifter 23a, and mixing it with the RF input signal; first and a second band pass filters 24a and 24b for filtering the signals outputted from the first and second mixers 22a, 22b respectively; a third mixer 25a for mixing an output signal of the first band pass filter 24a with an output signal of a second frequency oscillator 21b; a fourth mixer 25b for receiving a signal from the second frequency oscillator 21b that has been phase shifted by π/2 by the second phase-shifter 23b, and mixing it with an output signal of the second band pass filter 24b; a subtracter 26 for subtracting an output signal of the fourth mixer 25b from an output signal of the third mixer 25c; a fifth mixer 28 for mixing the output signal of the subtracter 26 with the output signal of an intermediate frequency oscillator 27, and down-converting a frequency of the signal; and a low-pass filter 29 for outputting a desired signal by passing a low band of the output signal of the fifth mixer 28.
The operation of the receiver having the Weaver structure will now be described.
A RF signal inputted to the receiver is divided into an I-signal and a Q-signal by using the first frequency oscillator 21a and the first phase-shifter 23a. The signals are respectively mixed with a ω1 frequency signal outputted from the first frequency oscillator 21a in the first and second mixers 22a, 22b and are down-converted into a ωRF−ω1 frequency. Because the I-signal (A) and the Q-signal (B) respectively outputted from the first and second mixers 22a, 22b pass the first and second band pass filters 24a, 24b, a second image signal, that is generated and is centered around a second frequency ω2, is rejected. Herein, the second image signal is an image signal generated during the down conversion of the signal into the second frequency ω2.
The output signals (C and D) of the first and second band pass filters 24a, 24b are respectively down-converted in the third and fourth mixers 25a, 25b with the signal of the second frequency oscillator 21b. An output (E) of the third mixer 25a is subtracted from an output signal (F) of the fourth mixer 25b in order to remove an image signal that is centered around a ωRF−ω1−ω2 frequency. An output signal (G) of the subtracter 26 is inputted to the fifth mixer 28 with the output signal of the intermediate frequency oscillator 27, is down-converted, passed through the low-pass filter 29 and is outputted as a base band signal.
When a RF input signal inputted to the receiver is ARF cos(ω1+ω2+ωIF)t+AIM1 cos(ω1−ω2−ωIF)t+AIM2 cos(ω1+ω2−ωIF)t , an output signal of the first frequency oscillator 21a is ALO(cos ω1t) and an output signal of the second frequency oscillator 21b is cos ω2t. The equations below represent the signals at points A–G in FIG. 3:
  A  ⁢      :      ⁢                    A        RF            ⁢              A        LO                    2      ⁢                            ⁢          ⁢      (                            cos          ⁡                      (                                          2                ⁢                                                                  ⁢                                  ω                  1                                            +                              ω                2                            +                              ω                IF                                      )                          ⁢        t            +              cos        ⁢                                  ⁢                  (                                    ω              2                        +                                          ω                IF                            ⁢              t                                )                    +                                                  A              IM1                        ⁢                          A              LO                                2                ⁢                  (                                                    cos                ⁡                                  (                                                            2                      ⁢                                              ω                        1                                                              -                                          ω                      2                                        -                                          ω                      IF                                                        )                                            ⁢              t                        +                          cos              ⁢                                                          ⁢                              (                                                      ω                    2                                    +                                      ω                    IF                                                  )                            ⁢              t                                )                    +                                                  A              IM2                        ⁢                          A              LO                                2                ⁢                  (                                                    cos                ⁡                                  (                                                            2                      ⁢                                              ω                        1                                                              +                                          ω                      2                                        -                                          ω                      IF                                                        )                                            ⁢              t                        +                          cos              ⁢                                                          ⁢                              (                                                      ω                    2                                                                                                                      ⁢                                          -                                              ω                        IF                                                                                            )                            ⁢              t                                )                ⁢                                  ⁢        B        ⁢                  :                  ⁢                                            A              RF                        ⁢                          A              LO                                            2            ⁢                                                                ⁢                                  ⁢                  (                                                    sin                ⁡                                  (                                                            2                      ⁢                                              ω                        1                                                              +                                          ω                      2                                        +                                          ω                      IF                                                        )                                            ⁢              t                        -                          sin              ⁢                                                          ⁢                              (                                                      ω                    2                                    +                                                            ω                      IF                                        ⁢                    t                                                  )                                      +                                                                                A                    IM1                                    ⁢                                      A                    LO                                                  2                            ⁢                              (                                                                            sin                      ⁡                                              (                                                                              2                            ⁢                                                          ω                              1                                                                                -                                                      ω                            2                                                    -                                                      ω                            IF                                                                          )                                                              ⁢                    t                                    +                                      sin                    ⁢                                                                                  ⁢                                          (                                                                        ω                          2                                                +                                                  ω                          IF                                                                    )                                        ⁢                    t                                                  )                                      +                                                                                A                    IM2                                    ⁢                                      A                    LO                                                  2                            ⁢                              (                                                                            sin                      ⁡                                              (                                                                              2                            ⁢                                                          ω                              1                                                                                +                                                      ω                            2                                                    -                                                      ω                            IF                                                                          )                                                              ⁢                    t                                    -                                      sin                    ⁢                                                                                  ⁢                                          (                                                                        ω                          2                                                -                                                  ω                          IF                                                                    )                                        ⁢                    t                                                  )                            ⁢                                                          ⁢              C              ⁢                              :                              ⁢                                                                    A                    RF                                    ⁢                                      A                    LO                                                  2                            ⁢                              (                                                      cos                    ⁡                                          (                                                                        ω                          2                                                +                                                                              ω                            IF                                                    ⁢                          t                                                                    )                                                        +                                                                                                              A                          IM1                                                ⁢                                                  A                          LO                                                                    2                                        ⁢                                          (                                                                        cos                          ⁡                                                      (                                                                                          ω                                2                                                            +                                                              ω                                IF                                                                                      )                                                                          ⁢                        t                                            )                                        ⁢                                                                                  ⁢                    D                    ⁢                                          :                                                          -                                                                                                              A                          RF                                                ⁢                                                  A                          LO                                                                    2                                        ⁢                                          (                                                                        sin                          ⁡                                                      (                                                                                          ω                                2                                                            +                                                                                                ω                                  IF                                                                ⁢                                t                                                                                      )                                                                          +                                                                                                                                            A                                IM1                                                            ⁢                                                              A                                LO                                                                                      2                                                    ⁢                                                      (                                                                                          sin                                ⁡                                                                  (                                                                                                            ω                                      2                                                                        +                                                                          ω                                      IF                                                                                                        )                                                                                            ⁢                              t                                                        )                                                    ⁢                                                                                                          ⁢                          E                          ⁢                                                      :                                                      ⁢                                                                                                                    A                                RF                                                            ⁢                                                              A                                LO                                                                                      4                                                    ⁢                                                      (                                                                                                                            cos                                  ⁡                                                                      (                                                                                                                  2                                        ⁢                                                                                  ω                                          2                                                                                                                    +                                                                              ω                                        IF                                                                                                              )                                                                                                  ⁢                                t                                                            +                                                              cos                                ⁢                                                                                                                                  ⁢                                                                  ω                                  IF                                                                ⁢                                t                                                                                      )                                                                          +                                                                                                                                            A                                IM1                                                            ⁢                                                              A                                LO                                                                                      4                                                    ⁢                                                      (                                                                                                                            cos                                  ⁡                                                                      (                                                                                                                  2                                        ⁢                                                                                  ω                                          2                                                                                                                    +                                                                              ω                                        IF                                                                                                              )                                                                                                  ⁢                                t                                                            +                                                              cos                                ⁢                                                                                                                                  ⁢                                                                  ω                                  IF                                                                ⁢                                t                                                                                      )                                                    ⁢                                                                                                          ⁢                          F                          ⁢                                                      :                                                      ⁢                                                                                                                    A                                RF                                                            ⁢                                                              A                                LO                                                                                      4                                                    ⁢                                                      (                                                                                                                            cos                                  ⁡                                                                      (                                                                                                                  2                                        ⁢                                                                                  ω                                          2                                                                                                                    +                                                                              ω                                        IF                                                                                                              )                                                                                                  ⁢                                t                                                            -                                                              cos                                ⁢                                                                                                                                  ⁢                                                                  ω                                  IF                                                                ⁢                                t                                                                                      )                                                                          -                                                                                                                                            A                                IM1                                                            ⁢                                                              A                                LO                                                                                      4                                                    ⁢                                                                                                          ⁢                                                      (                                                                                                                            cos                                  ⁡                                                                      (                                                                                                                  2                                        ⁢                                                                                  ω                                          2                                                                                                                    +                                                                              ω                                        IF                                                                                                              )                                                                                                  ⁢                                t                                                            -                                                              cos                                ⁢                                                                                                                                  ⁢                                                                  ω                                  IF                                                                ⁢                                t                                                                                      )                                                    ⁢                                                                                                          ⁢                          G                          ⁢                                                      :                                                      ⁢                                                                                                                    A                                RF                                                            ⁢                                                              A                                LO                                                                                      2                                                    ⁢                          cos                          ⁢                                                                                                          ⁢                                                      ω                            IF                                                    ⁢                          t                                                +                                                                                                                                            A                                IM1                                                            ⁢                                                              A                                LO                                                                                      2                                                    ⁢                                                      cos                            ⁡                                                          (                                                                                                2                                  ⁢                                                                      ω                                    2                                                                                                  +                                                                  ω                                  IF                                                                                            )                                                                                ⁢                          t                                                                                                                                                            
The related art receiver having the Hartley structure implements a π/2 phase-shifter by using resistance and a capacitor. Because the resistance and a capacity of the capacitor may be varied according to a frequency and the surroundings, phase imbalance may occur in the phase-shifter, which can result in a gain mismatch. Accordingly, it is difficult to reject an image signal accurately.
In the related art receiver having a Weaver structure, because a π/2 phase shifter is not used, problems such as phase imbalance or gain mismatch do not occur. However, other problems, such as a second image occurrence and harmonics due to non-linearity in a mixing process after a low-pass filter are present. Thus, it is difficult to match a phase and a size of the signals accurately.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.